Example embodiments of the inventive concepts generally relate to a magneto-resistive device including magnetic tunnel junctions, and in particular, to a magnetic tunnel device using a spin transfer magnetization inversion technique and a magneto-resistive device of a magneto-resistive random access memory (MRAM).
A magneto-resistive random access memory (MRAM) is an emerging nonvolatile memory device with excellent properties, such as high speed, low power consumption, and large capacity. In the magneto-resistive memory, a magneto-resistive device with a magnetic tunnel junction (MTJ) is used as a memory element. For example, the magneto-resistive device includes a free layer whose magnetization direction can be changed, a pinned layer whose magnetization direction is fixed to be perpendicular to a surface thereof, and a tunnel barrier layer that is interposed between the free and pinned layers and is formed of an insulating material. The pinned layer, the tunnel barrier layer, and the free layer constitute the magnetic tunnel junction. The magneto-resistive device may be referred to as a “MTJ device”.
For the magneto-resistive device, a relative magnetization direction of a pair of magnetic layers separated from each other by the intervening tunnel barrier layer may be adjusted to change magneto-resistance of the magneto-resistive device and store data in the magneto-resistive device. In the magneto-resistive device, a read operation may be performed using such a magneto-resistive effect, and a writing operation may be performed using a spin transfer magnetization inversion technique (e.g., a spin transfer torque (STT) technique).
Ferromagnetic materials with high perpendicular magnetic anisotropy and high spin polarizability are preferred as materials for the pinned and free layers.
In the STT technique for the writing operation, a spin magnetic moment of electrons may be used to invert a magnetization direction of the free layer. The use of the STT technique may makes it possible to more effectively realize a magneto-resistive device with a fine pattern size and a low current property, compared with the conventional method of flowing electric current through a wire. Furthermore, the magneto-resistive device is resistant to thermal disturbance caused by a reduction in pattern size.
The magneto-resistive device is expected to be used as a basic component for a next-generation high density memory device, such as a STT-MRAM.
CoFeB-based materials are extensively used as a material for the free layer. However, for CoFeB-based materials, since the perpendicular magnetic anisotropy is realized using an interface magnetic anisotropy, the use of the CoFeB-based materials may lead to a low perpendicular magnetic anisotropy. In addition, there is a very narrow range in known materials for the free layer.
An alternative method of providing a perpendicular-magnetization preserving layer in MTJs has been proposed. For example, Patent Document 1 (Japanese Patent Publication No. 2005-032878) describes a magneto-resistive device including a magnetization pinned layer whose spin magnetic moment is fixed and perpendicular to a surface thereof, a magnetic writing layer whose spin moment is perpendicular to a surface thereof, a non-magnetic layer provided between the magnetization pinned layer and the magnetic writing layer, and an anti-ferromagnetic layer provided on at least a surface of the magnetization pinned layer.
Also, Patent Document 2 (Japanese Patent Publication No. 2005-150303) describes a magneto-resistive device, in which a ferromagnetic tunnel junction including three layers (e.g., a first ferromagnetic layer, a tunnel barrier layer, and a second ferromagnetic layer) is provided. Here, the first ferromagnetic layer has a coercive force greater than that of the second ferromagnetic layer, and tunnel conductance is changed depending on a relative angle between magnetizations of the first and second ferromagnetic layers. Magnetization of an end portion of the second ferromagnetic layer is fixed and perpendicular with respect to a direction of a magnetization easy axis of the second ferromagnetic layer.
Also, Patent Document 3 (Japanese Patent Publication No. 2011-071352) describes a magneto-resistive device including a first magnetic layer having a magnetization easy axis perpendicular to a surface thereof and a variable magnetization direction, a second magnetic layer having a magnetization easy axis perpendicular to a surface thereof and a fixed magnetization direction, and a first non-magnetic layer provided between the first and second magnetic layers. The first magnetic layer is formed of a ferromagnetic material containing a CoPd or CoPt alloy, in which Co/Pd or Co/Pt layers are alternately stacked on a closed packed plane, and having a c-axis perpendicular to a surface thereof. A magnetization direction of the first magnetic layer is changed by a bi-directional current flowing through the first magnetic layer, the first non-magnetic layer, and the second magnetic layer.
By using the above techniques, it becomes possible to select a material with high spin polarizability from a group including half-metal and Heusler based materials. However, this may lead to an increase in layer thickness of a device and consequently an increase in magnetization switching current and power consumption.
To settle such a problem, a magneto-resistive device capable of reducing a magnetization switching current and providing a low power consumption property is proposed in, for example, Patent Document 4 (Japanese Patent Publication No. 2014-116474). According to the magneto-resistive device described in the Patent Document 4, a memory layer (or a free layer) has a ferromagnetic layer, a perpendicular magnetization preserving layer, and a magnetic-coupling control layer. The magnetic-coupling control layer is provided between the ferromagnetic layer and the perpendicular magnetization preserving layer to control magnetic coupling between the ferromagnetic layer and the perpendicular magnetization preserving layer. A thickness of the magnetic-coupling control layer may be changed to optimize various parameters (e.g., a changing rate of resistance, thermal stability, a write current, and a magnetization switching speed) and reduce a current required for a magnetization switching operation.